The 3GPP standard Long-Term-Evolution (LTE) of UTRAN is a system using orthogonal frequency division multiplex (OFDM) standards with frequency-localized allocations.
In the LTE system one main difference to earlier 3GPP releases is the use of wide channels that are shared with users in frequency domain (i.e., frequency division multiplexing). Allocation for one user can vary from one physical resource block (PRB) to maximum number of resource blocks in the channel (e.g. 50 PRB for 10 MHz channel). A physical resource block is the smallest allocable frequency range of the uplink or downlink frequency band lasting a predefined time. E.g., in LTE, a physical resource block is 180 kHz wide and lasts for a 0.5 ms time slot (transmission time interval, TTI).
It is common understanding in the industry that LTE—in the long run—will completely substitute incumbent radio access technologies including Code Division Multiplex Access (CDMA), Wideband CDMA (W-CDMA), and Global System for Mobile communication (GSM)/Enhanced Data rate for GSM Evolution (EDGE) where spectrum allocations and regulatory rules will allow for.
It is therefore of vital interest to operators, that migration e.g. from CDMA or from W-CDMA to LTE is possible without the need for further increasing the number of Base Station sites or the amount or size of antenna configurations.
Hence, for the same traffic profile and demand, LTE link budgets shall match those of CDMA or W-CDMA. Further, LTE link budgets matching conventional traffic profiles and demand must also match LTE link budgets for new applications like down streaming or heavy downloading.
The LTE downlink (DL) link budget is critical for the Physical Downlink Shared Channel (PDSCH) with high data rate services like video or multi-media streaming (e.g. ½ Mbps at the cell edge), and for low data rate services like VoIP (e.g. 5.9 Kbps or 12.2/12.8 Kbps) at the cell edge.
The communication link between an evolved NodeB (eNB) and an user equipment (UE) is created through a set of channels. For downlink communication, the UE is notified of an incoming data packet through the physical downlink control channel (PDCCH), and the user payload is carried on the physical downlink shared channel (PDSCH), which can be used to carry traffic to multiple UEs (frequency multiplexed) within the same transmit time interval (TTI).
Similarly, uplink traffic will also be scheduled through the PDCCH (but with an uplink grant), and the physical uplink shared channel (PUSCH) will be used for the transmission of the data from the UE to the eNB.
One issue that has been observed when comparing the LTE system to the WCDMA system is the fact that LTE is lacking some power adjustment flexibility in terms of providing all the system power to a single UE with low data rates. For WCDMA/HSDPA, it is possible to have a single user using all the transmit power, while this is not possible for LTE release 8 for relative small user payloads.
Several methods have been proposed to enhance the PDSCH coverage. For example, for a critical UE/service, as many physical resource blocks (PRB) as possible for a given transport block and an as robust modulation and coding scheme (MCS) may be allocated. Alternatively, some energy may be “borrowed” from PRBs that are using less than the nominal eNB TX power or are not used at all (see 3GPP TS 36.104 v8.5.0, section 6.3.1.1).
Current mechanisms for adapting the performance of the PDCCH are providing power and coding control of the PDCCH. This is achieved through (1) aggregation of the control channel elements (CCEs), and (2) power boosting of the PDCCH (by shifting power from unused CCEs to CCEs used for poor coverage UEs).
One problem of the first approach is that the maximum “legal” number of CCEs to aggregate for a PDCCH will be 8, which is currently well balanced to the current performance of the standard Rel'8 PDSCH. Further, power boosting may be used, but according to 3GPP hardware specifications (to reduce the power leakage to other frequencies), it is only allowed to boost the transmit power by 4 dB (a bit more than a factor of 2).
Therefore, there does not seem to be a balance between the extension methods of the PDSCH and the PDCCH.